add wrapper for git_config_open_default (#758)

config.go

@@ -450,3 +450,17 @@ func ConfigFindProgramdata() (string, error) {
 
 	return C.GoString(buf.ptr), nil
 }
+
+// OpenDefault opens the default config according to git rules
+func OpenDefault() (*Config, error) {
+	runtime.LockOSThread()
+	defer runtime.UnlockOSThread()
+
+	config := new(Config)
+
+	if ret := C.git_config_open_default(&config.ptr); ret < 0 {
+		return nil, MakeGitError(ret)
+	}
+
+	return config, nil
+}

config_test.go

@@ -107,3 +107,13 @@ func TestConfigLookups(t *testing.T) {
 		test(c, t)
 	}
 }
+
+func TestOpenDefault(t *testing.T) {
+
+	c, err := OpenDefault()
+	if err != nil {
+		t.Errorf("OpenDefault error: '%v'. Expected none\n", err)
+		return
+	}
+	defer c.Free()
+}